1. Field of the Invention
The invention relates to an optical device for combining at least two sub-beams of different colours to one colour beam, which device comprises at least one colour-selective reflector arranged on a transparent plate-shaped substrate and which is further provided with at least one correction element arranged in the path of a sub-beam, which has traversed the substrate, for compensating astigmatism introduced by this substrate in the relevant sub-beam.
A device of this type may be used, for example in a colour image projection apparatus for combining three sub-beams emitted by three primary image sources generating three monochromatic images, each in a different colour, of one and the same scene to one colour beam which is applied to one projection lens system. Thus, the three monochromatic images can be projected as one image on a projection screen.
2. Discussion of the Related Art
A device for combining three differently coloured sub-beams emitted by three liquid crystalline picture display panels, hereinafter referred to as LCD panels, is described, inter alia in U.S. Pat. No. 5,046,837. This device comprises two colour-selective reflectors in the form of dichroic mirrors, each arranged on a glass substrate. One of the sub-beams, for example the green beam is reflected by the two mirrors while the two other sub-beams, the red and the blue beam are reflected once by a dichroic mirror and pass the substrate of a mirror once. Since these mirrors are arranged obliquely in the three beams, for example at an angle of 45.degree. to the axis of the sub-beams and the sub-beams are converging beams at the area of the mirrors, a sub-beam passing through the substrate of a mirror will become astigmatic, which means that this beam has different convergences in two mutually perpendicular planes comprising the beam axis. Consequently, the beam will not be sharply focused on the projection screen in said two planes so that the projected image exhibits an aberration. Since the mirrors are arranged in an imaging optical system, they should have a high degree of planeness, which can only be achieved by making use of relatively thick substrates so that said astigmatism is relatively large. This astigmatism only occurs in two of the three sub-beams. Consequently, the distortion in the projected colour image cannot be corrected by one correction element in the path of the combined beam.
In (JP-A) 3-170925 it is proposed to eliminate the aberration by arranging a cylindrical lens on each of the mirror substrates opposite the dichroic layer. As is known, a cylindrical lens introduces astigmatism in a beam. By suitable choice of the power and position of the cylindrical lens, the astigmatism caused thereby may compensate the astigmatism caused by an oblique passage of a beam through a plate. However, only the astigmatism and no other aberration produced upon passage through a plate, inter alia coma, can be corrected with such a cylindrical lens.